Hazardous Environment Combat Unit (HECU) is a branch of the united States Marine Corps special forces.
The HECU Marines are sent in to clean up the Black Mesa Research Facility following the incident, their mission being to eliminate anyone with knowledge of the research project and the extraterrestrial life involved. They are specially trained at the Santego Military Base to deal with a variety of indoor combat situations, particularly those involving dangerous environments and unconventional hostiles. Apart from combat roles, they make take on specialized capacities such as becoming Medics or Engineers.
Works wonders with "Modern Firearms"